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Abstract:
The genetic determination of sex requires either non-recombining chromosome regions or complete sex chromosomes, both of which have evolved independently and repeatedly across different eukaryotic species. While the processes shaping the evolution of sex chromosomes are increasingly well understood in diploid organisms, haploid sex determination systems (U/V) have been under-studied. We have recently sequenced and analysed the sex determining region of the brown alga Ectocarpus, where sex is expressed in the gametophyte generation, during the haploid phase of the life cycle, and both the female (U) and the male (V) sex chromosomes contain sex-determining regions (SDRs; Ahmed et al. 2014, Current Biology). Here, we use experimental and theoretical approaches to investigate the extensive pseudoautosomal regions (PARs) that border the SDR on the Ectocarpus sex chromosome. Despite a considerable amount of theoretical work on PAR genetics and evolution, these genomic regions have remained poorly characterized empirically, even in classic model organisms. We show that although the PARs of the U/V sex chromosomes of Ectocarpus recombine at a similar rate to autosomal regions of the genome, they exhibit many features typical of non-recombining regions. The pseudoautosomal regions also preferentially accumulate sporophyte-biased genes, which tend to occur in physically linked clusters. A modelling-based approach was used to investigate possible evolutionary mechanisms underlying this enrichment in sporophyte-biased genes. We provide the first detailed analysis of the recombining regions of a haploid sex chromosome system and propose a mechanism that may explain some of the exceptional evolutionary features of these regions compared with autosomes.
